1. Field of the Invention
The present invention relates to computerized simulation of hydrocarbon reservoirs in the earth, and in particular to simulation and forecasting of production from such reservoirs with determination of phase equilibrium conditions in cells of the reservoirs.
2. Description of the Related Art
The early development of compositional reservoir simulators in the industry was, so far as is known, restricted to reservoir models small enough to be characterized by a relatively small number of cells (of the order of 100,000) into which the reservoir of interest was organized. Models of this early type provided adequate numerical resolution for small to medium size reservoirs or fields.
The early models became too coarse in data content and accuracy for what have become known as giant oil and gas fields. Giant reservoirs are those mammoth subsurface reservoirs at various locations on the earth containing hydrocarbons and other fluids. In giant reservoirs, there may be thousands of wells, and possibly hundreds of well groups, with tens of thousands of completions, when the total number of wells is considered. For giant reservoirs, the sheer volume of the data involved becomes a problem in simulation and analysis of performance over a period of time.
The problem caused by the volume of data for a giant reservoir has been even more the case when results are simulated for reservoir performance at each of a number of dates over a period of time, such as the expected production life in years for the reservoir. As a result of this, sufficient cell resolution was possible in some instances. However this required a compromise at the expense of dividing the giant reservoir model into separate sectors. The data for the different, separate sectors were then each separately processed.
Compositional reservoir simulation also required fast and accurate computation of phase equilibrium compositions of the fluids in the reservoir. Typical reservoir simulation was predicated on the presence in the formation fluids of a number of hydrocarbon components, which could be on the order of from eight to seventeen, based upon laboratory fluid characterization, as well as water.
There has also been increased interest in reservoir analysis for taking into account enhanced oil recovery methods and CO2 sequestration. In order that simulation results for this purpose be accurate, inorganic components (such as nitrogen, CO2, sulfides, for example) had to be included along with the hydrocarbons and water as reservoir fluids which would be present in the reservoirs as a result of these processes. Inclusion of inorganic components into the reservoir simulation process thus added to the already large number of reservoir hydrocarbon components and water.
Simulation of fluid composition and behavior over time in a reservoir has been governed by an equation of state (or EOS) determination for each of the multiple component hydrocarbons and water which were present in the reservoir cells in various proportions. As mentioned, an EOS determination was made for each of the reservoir cells during reservoir simulation at a number of spaced time intervals over the projected reservoir productive life. Depending on pressure changes and the like over the projected reservoir life, the relative percentages of hydrocarbons present and the relative amounts of gaseous phase and liquid or vapor phase also had to be taken into account.
Another area of increasing recent interest has been in the area of online/interactive reservoir simulation to monitor producing oil and gas fields, also referred to as I-Field technology. U.S. Published Patent Application No, 2009/0276100, commonly owned with the present application, is an example of this type of real-time reservoir management. For online/interactive simulation of this type to be meaningful and effective, it was important that the reservoir simulator produced accurate results at a speed which would keep pace with the rate of real-time acquisition of field measurement data from the reservoir.
U.S. Pat. No. 7,526,418, which is assigned to the assignee of the present application, provided a simulator for dealing with certain aspects of giant reservoir models. In doing so, reservoirs were organized into cells which were organized into a model having millions of cells. Although significant reduction in processing time and more accurate data resolution were provides, recent needs in the art have increased data processing speed and accuracy requirements in reservoir simulation.
Examples of other simplifications and compromise measures which have been taken in other contexts to avoid extensive computation or processing computer time are shown in the prior art. U.S. Pat. No. 7,548,840 is an example of efforts save computer time by reducing the number of variables (components) to be computed using approximations.
When application to multi-million (or billion) cell compositional reservoir simulation was not intended, then slow conventional methods can be tolerated in many cases. For example, when only surface facilities, and not reservoirs, are involved (as in production allocation systems of commingled fluid streams), commercial (CPU-based) process simulators have generally been quite adequate. U.S. Pat. No. 7,373,285 is an example of such an application where processing speed was not a concern.